C6. Insects of the wider countryside (butterflies)

a. Semi-natural habitat specialists

b. Species of the wider countryside

 

Type: State Indicator

 

Indicator Description

This indicator consists of 2 measures of annual butterfly population abundance: the first for specialist butterflies (species strongly associated with semi-natural habitats such as unimproved grassland) and the second for butterflies found in both semi-natural habitats and the wider countryside. 

Butterflies are complementary to birds and bats as an indicator, especially the habitat specialists, because they use resources in the landscape at a much finer spatial scale than either of these groups.

 

Summary

 

butterfly Since 1976, the habitat specialists butterflies index has fallen by 77%.

butterfly Over the same period, the index for species of the wider countryside has fallen by 46%.

butterfly Large fluctuations in numbers between years are typical features of butterfly populations, principally in response to weather conditions. 2017 was a relatively bad year for butterflies; it was likely due to periods of unfavourable weather during the spring and summer months.

butterfly The statistical assessment of change is made on an analysis of the underlying smoothed trends.  Since 1976, populations of habitat specialists and species of the wider countryside have declined significantly but both trends show no significant change since 2012.

 

Habitat Specialists

 

Figure C6ai.  Trends in butterfly populations in the UK: habitat specialists, 1976 to 2017.

 

Figure C6ai. Trends in butterfly populations in the UK: habitat specialists, 1976 to 2017.

 

Notes:

  1. The figure in brackets shows the number of species included in the index.
  2. The graph shows the unsmoothed trend (dashed line) and smoothed trend (solid line) together with its 95% confidence interval (shaded).
  3. The bar chart shows the percentage of species within the indicator that have shown a statistically significant increase, a statistically significant decrease or shown no significant change.
  4.  In 2018, an improved analysis method was used to derive the species indices (see ‘Background’ section for further information). The graph is therefore not directly comparable to those in previous versions of this publication.

 

Source: Butterfly Conservation, Centre for Ecology & Hydrology, Defra, Joint Nature Conservation Committee.

 

Habitat specialist species, which are vulnerable to semi-natural habitat loss and fragmentation, have not recovered from population declines experienced in the late 1970s, attributed mainly to the knock-on effects of the drought conditions experienced in 1976.  The habitat specialist index declined by 77% between 1976 and 2017 (Figure C6ai).  Underlying analysis shows that this decrease was due to a statistically significant reduction in relative abundance over the period 1976 to 2000 that was more pronounced in the late 1970s and early 1980s. The index showed an increase over the period 2012 to 2017, from 19% to 23% of the 1977 level.  However, this change is not statistically significant and an amber assessment is assigned to the measure in the short term. 

Species fare differently within the overall trend.  Habitat specialists  showing the greatest decline since 1976 include: heath fritillary, wood white, Lulworth skipper and pearl-bordered fritillary.  Grayling, grizzled skipper and large heath show a decline in the short term since 2012. Silver-spotted skipper, dark green fritillary, large heath, adonis blue and silver-washed fritillary show significant increases over the long term, whilst black hairstreak is the only habitat specialist to show a statistically significant increase since 2012.

 

Species of the wider countryside

 

Figure C6bi.  Trends in butterfly populations in the UK: species of the wider countryside, 1976 to 2017.

 

Figure C6bi. Trends in butterfly populations in the UK: species of the wider countryside, 1976 to 2017.

 

 

Notes:

  1. The figure in brackets shows the number of species included in the index.
  2. This indicator includes individual measures for 25 species of butterflies; the wider countryside index, however, only includes 24 trends. This is because an aggregate trend is used for small skipper (Thymelicus lineola) and Essex skipper (Thymelicus sylvestris); these 2 species have been combined due to historical difficulties with distinguishing them in the field.
  3. The graph shows the unsmoothed trend (dashed line) and smoothed trend (solid line) together with its 95% confidence interval (shaded).
  4.  The bar chart shows the percentage of species within the indicator that have shown a statistically significant increase, a statistically significant decrease or shown no significant change.
  5.  In 2018, an improved analysis method was used to derive the species indices (see ‘Background’ section for further information). The graph is therefore not directly comparable to those in previous versions of this publication.

Source: British Trust for Ornithology, Butterfly Conservation, Centre for Ecology & Hydrology, Defra, Joint Nature Conservation Committee.

The species of the wider countryside index decreased by 46% between 1976 and 2017 (Figure C6bi); the underlying analysis indicates that this decrease was chiefly due to a statistically significant reduction in abundance over the period 1976 to 2006.  The index showed an increase over the period 2012 to 2017, from 36% to 54% of the 1976 level, however, this change is not statistically significant and an amber assessment is assigned to the measure in the short term.

Species again fare differently within the overall trend.  Species of the wider countryside showing the greatest declines since 1976 include: white-letter hairstreak, wall, and small tortoiseshell; Scotch Argus was the only species in this index to show a decline in the short term since 2012.  Comma, marbled white, speckled wood and ringlet show increases over the long term; no species show a short-term increase since 2012.

 

Assessment of change in butterfly populations

 

Long term

Short term

Latest year

Semi-natural habitat specialists

indicator declining
1976–2017

indicator stable
2012–2017

 

Increased (2017)

 

Species of the wider countryside

indicator declining
1976–2017

indicator stable
2012–2017

Increased  (2017)

 

Notes:

While percentage changes in these indices are reported based on the most recent unsmoothed data point (2017), the formal long-term and short-term assessments of the statistical significance of these changes are made using the smoothed data to 2017.  Analysis of the underlying trends is undertaken by the data providers. 

 

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Last updated: July 2018

Latest data available: 2017